Metal ion reduction in top anti-reflective coatings for photoresists

ABSTRACT

The present invention provides methods for producing top anti-reflective coating compositions having a very low level of metal ions, utilizing specially treated ion exchange resins. A method is also provided for producing semiconductor devices using such top anti-reflective coating compositions.

This is a continuation of application Ser. No. 07/984,655 filed on Dec.2, 1992, now abandoned, which is a continuation-in-part of Ser. No.07/911,604 filed Jul. 10, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing topanti-reflective coatings for photoresists, which coatings have a verylow level of metal ions, especially sodium and iron, and for using suchtop anti-reflective coatings with light-sensitive photoresistcompositions to produce semiconductor devices. Further, the presentinvention relates to a process for coating substrates already coatedwith a photoresist composition with these top anti-reflective coatingcompositions, as well as the process of coating, imaging and developinglight-sensitive photoresist compositions coated with suchanti-reflective coatings on such substrates.

Thin film interference plays a central role in the process control ofoptical microlithography. Small variations in the thickness of resist orof thin films underneath the resist cause large exposure variations,which in turn cause two classes of undesirable line width variations.

1. As thin film thickness may vary from run to run, wafer to wafer, oracross a wafer, line widths will vary from run to run, wafer to wafer oracross a wafer.

2. As patterning takes place over wafer topography, the resist thicknessunavoidably changes at the topography edge causing the line width tovary as it crosses the edge.

Avoiding such thin film interference effects is one of the keyadvantages of advanced processes such as X-Ray lithography ormulti-layer resist systems. However, Single Layer Resist (SLR) processesdominate semiconductor manufacturing lines because of the theirsimplicity and cost-effectiveness, and also because of the relativecleanliness of wet developed processes compared with dry processes.

Thin film interference results in periodic undulations in a plot of theexposure dose required to clear positive photoresist (termeddose-to-clear) versus the photoresist thickness. Optically, on aresist-coated substrate, light reflected from the bottom mirror (due tothe effect of the substrate+thin films) interferes with the refection ofthe top mirror (the resist/air interface).

As optical lithography pushes towards shorter wavelengths, thin filminterference effects become increasingly important. More severe swingsin intensity are seen as wavelength decreases.

One strategy for reducing thin film interference is to reduce thesubstrate reflectivity through the use of absorptive Anti-ReflectiveCoats. One way of doing this is to apply a Top Anti-Reflector coating ontop of the photoresist prior to exposure.

Photoresist compositions are used in microlithography processes formaking miniaturized electronic components such as in the fabrication ofcomputer chips and integrated circuits. Generally, in these processes, athin coating of film of a photoresist composition is first applied to asubstrate material, such as silicon wafers used for making integratedcircuits. The coated substrate is then baked to evaporate any solvent inthe photoresist composition and to fix the coating onto the substrate.The baked coated surface of the substrate is next subjected to animage-wise exposure to radiation.

This radiation exposure causes a chemical transformation in the exposedareas of the coated surface. Visible light, ultraviolet (UV) light,electron beam and X-ray radiant energy are radiation types commonly usedtoday in microlithographic processes. After this image-wise exposure,the coated substrate is treated with a developer solution to dissolveand remove either the radiation-exposed or the unexposed areas of thephotoresist and all of the anti-reflective coating from the surface ofthe substrate.

Metal contamination has been a problem for a long time in thefabrication of high density integrated circuits and computer chips,often leading to increased defects, yield losses, degradation anddecreased performance. In plasma processes, metals such as sodium andiron, when they are present in the photoresist or in a coating on thephotoresist, can cause contamination especially during plasma stripping.However, these problems have been overcome to a substantial extentduring the fabrication process, for example, by utilizing HCL gatheringof the contaminants during a high temperature anneal cycle.

As semiconductor devices have become more sophisticated, these problemshave become much more difficult to overcome. When silicon wafers arecoated with a liquid positive photoresist and subsequently stripped off,such as with oxygen microwave plasma, the performance and stability ofthe semiconductor device is often seen to decrease. As the plasmastripping process is repeated, more degradation of the device frequentlyoccurs. A primary cause of such problems can be the metal contaminationin the anti-reflective coating on the photoresist, particularly sodiumand iron ions. Metal levels of as low as 1.0 ppm or less can adverselyaffect the properties of such semiconductor devices.

There are two types of photoresist compositions, negative-working andpositive-working. When negative-working photoresist compositions areexposed image-wise to radiation, the areas of the resist compositionexposed to the radiation become less soluble to a developer solution(e.g. a cross-linking reaction occurs) while the unexposed areas of thephotoresist coating remain relatively soluble to such a solution. Thus,treatment of an exposed negative-working resist with a developer causesremoval of the non-exposed areas of the photoresist coating and thecreation of a negative image in the coating. Thereby uncovering adesired portion of the underlying substrate surface on which thephotoresist composition was deposited.

On the other hand, when positive-working photoresist compositions areexposed image-wise to radiation, those areas of the photoresistcomposition exposed to the radiation become more soluble in thedeveloper solution (e.g. a rearrangement reaction occurs) while thoseareas not exposed remain relatively insoluble in the developer solution.Thus, treatment of an exposed positive-working photoresist with thedeveloper causes removal of the exposed areas of the coating and thecreation of a positive image in the photoresist coating. Again, adesired portion of the underlying substrate surface is uncovered.

After this development operation, the now partially unprotectedsubstrate may be treated with a substrate-etchant solution or plasmagases and the like. The etchant solution or plasma gases etch thatportion of the substrate where the photoresist coating was removedduring development. The areas of the substrate where the photoresistcoating still remains are protected and, thus, an etched pattern iscreated in the substrate material which corresponds to the photomaskused for the image-wise exposure of the radiation. Later, the remainingareas of the photoresist coating may be removed during a strippingoperation, leaving a clean etched substrate surface. In some instances,it is desirable to heat treat the remaining photoresist layer, after thedevelopment step and before the etching step, to increase its adhesionto the underlying substrate and its resistance to etching solutions.

Positive working photoresist compositions are currently favored overnegative working resists because the former generally have betterresolution capabilities and pattern transfer characteristics.Photoresist resolution is defined as the smallest feature which theresist composition can transfer from the photomask to the substrate witha high degree of image edge acuity after exposure and development. Inmany manufacturing applications today, resist resolution on the order ofless then one micron are necessary. In addition, it is almost alwaysdesirable that the developed photoresist wall profiles be near verticalrelative to the substrate. Such demarcations between developed andundeveloped areas of the resist coating translate into accurate patterntransfer of the mask image onto the substrate.

SUMMARY OF THE INVENTION

The present invention relates to a process for producing topanti-reflective coatings containing very low levels of metal ions,especially sodium and iron. The invention further relates to a processfor producing semiconductor devices using such top anti-reflectivecoatings for photoresists.

The process of the subject invention provides a top anti-reflectivecoating having a very low level of metal ions. The anti-reflectivecoating is applied on top of a photoresist, which may be either negativeor positive working, although positive photoresists are preferred.

The top anti-reflective coatings obtained have very low levels of metalions such as iron, sodium, potassium, calcium, magnesium, copper andzinc. The total metal ion level is preferably less than 1 ppm, morepreferably less than 500 ppb. Sodium and iron are the most common metalion contaminants and among the easiest to detect. The level of thesemetal ions serves as an indicator of the level of other metal ions. Thelevel of sodium and iron ions, are each respectively, less than 200 ppb,preferably less than 100 ppb, more preferably less than 50 ppb, evenmore preferably less than 20 ppb and most preferably less than 10 ppb.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a process for producing a topanti-reflective coating having a very low level of metal ions,particularly sodium and iron. In the preferred embodiment, the processutilizes an acidic ion exchange resin to purify the top anti-reflectivecoating. The subject process comprises:

a) treating an acidic ion exchange resin with water, preferablydeionized water, followed by a mineral acid solution (e.g. a 5-98%solution of sulfuric, nitric or hydrochloric acid) to reduce the levelof sodium and iron ions in the ion exchange resin to less than 500 ppbeach, preferably less than 200 ppb, more preferably less than 100 ppband most preferably no more than 40 ppb;

b) providing a solution of 5 to 40 weight percent of a water solubleorganic carboxylic acid polymer having a molecular weight of from about500 to about 100,000, preferably from about 1,000 to about 10,000, in asuitable solvent;

c) passing the water soluble organic carboxylic acid polymer solutionthrough the ion exchange resin and reducing the level of total sodiumand iron ions in the solution to less than 200 ppb each, preferably lessthan 100 ppb, more preferably less than 50 ppb, even more preferablyless than 20 ppb and most preferably less than 10 ppb;

(d) formulating a top anti-reflective coating composition by providingan admixture of:

(1) the treated water soluble organic carboxylic acid;

(2) a fluorine containing water sparingly soluble (0.1%-10% by weight inwater, preferably 0.5%-5% by weight) organic (C₃ -C₁₃ ) aliphaticcarboxylic acid;

(3) an ammonium hydroxide; and

(4) a suitable solvent.

Prior to formulating the top anti-reflective coating composition, asolution of the fluorine containing sparingly water soluble organicaliphatic carboxylic acid in a suitable solvent is preferably passedthrough the ion exchange resin and the level of sodium and iron ions inthe solution reduced to less than 200 ppb each, preferably less than 100ppb, more preferably less than 50 ppb, even more preferably less than 20ppb and most preferably less than 10 ppb.

The solvents for the water soluble organic carboxylic acid, thesparingly water soluble halogen containing organic aliphatic carboxylicacid and the top anti-reflective coating are preferably deionized, e.g.deionized water or deionized diglyme or mixture of deionized water anddeionized diglyme.

Prior to formulating the final top anti-reflective coating, preferablyan admixture is provided of:

(1) the treated water soluble organic carboxylic acid;

(2) the treated halogen containing sparingly water-soluble organicaliphatic carboxylic acid; and

(3) a suitable solvent.

The admixture is then passed through the ion exchange resin and thelevel of total sodium and iron ions in the solution reduced to less than100 ppb each, preferably less than 50 ppb, more preferably less than 20ppb and most preferably less than 10 ppb. The ammonium hydroxide is thenadded to the admixture to provide a top antireflective coating having avery low level of metal ions.

Preferably, prior to the ion exchange resin treating of the watersoluble organic carboxylic acid, the sparingly water soluble halogencontaining organic aliphatic carboxylic acid or the admixture of thesetwo components, the ion exchange resin is treated with a solvent whichis the same as or at least compatible with the solvent for the componentor mixture of components which is to be treated with the ion exchangeresin. Most preferably, the ion exchange resin is treated withsufficient new solvent to substantially remove other solvents and tosaturate the ion exchange resin with the new solvent.

An acidic ion exchange resin, such as a styrene/divinylbenzene cationexchange resin, is utilized in the present process. Such ion exchangeresins are available from Rohm and Haas Company, e.g. AMBERLYST 15resin. These resins typically contain as much as 80,000 to 200,000 ppbeach of sodium and iron. Before utilized in the process of theinvention, the ion exchange resin must be treated with water and then amineral acid solution to reduce the metal ion level. Preferably the ionexchange resin is initially rinsed with deionized water, followed by amineral acid solution, such as a 10 percent sulfuric acid solution,rinsed again with deionized water, treated again with the mineral acidsolution and once more rinsed with deionized water. Before purifying theanti-reflective coating composition solution, it is critical that theion exchange resin is first rinsed with a solvent which is the same as,or at least compatible with, the antireflective coating compositionsolvent.

If the anti-reflective coating or any of its components contains one ormore constituents which will react chemically with the acidic ionexchange resin, the anti-reflective coating or component is preferablyinitially formulated without such constituents, e.g. the ammoniumhydroxide. This will provide an anti-reflective coating or componentsubstantially free of any constituents which will react with the acidicion exchange resin. After purification, such constituents are added tothe anit-reflective coating.

The solution of the anti-reflective coating or component is passedthrough a column containing the ion exchange resin, e.g. a solution offrom about 1 to 40 weight percent in a suitable solvent. Such solutionsmay typically contain from 500 to 20,000 ppb each of sodium and ironions. During the process of the present invention, these levels are eachreduced to as low as 10 ppb each, or less.

The present invention provides a process for producing a topanti-reflective coating having a very low level of metal ions and aprocess for producing semiconductor devices using such anti-reflectivecoatings. The anti-reflective coating is formed by providing anadmixture of a water soluble organic carboxylic acid, a sparinglywater-soluble halogen containing organic aliphatic carboxylic acid, anammonium hydroxide and a suitable solvent.

Suitable water soluble organic carboxylic acids include acrylic andmethacrylic acids, such as poly(acrylic acid) poly(methacrylic acid).Suitable sparingly water soluble fluorine containing organic aliphaticcarboxylic acids include fluorinated (₃ -C₁₈ aliphatic carboxylic acids,such as pentadecafluorooctanoic acid.

Suitable solvents, which are preferably deionized, include water,diglyme, propylene glycol monoethyl ether acetate (PGMEA), ethyllactate, ethyl-3-ethoxypropionate, mixtures of ethyl lactate andethyl-3-ethoxy proprionate, xylene, butyl acetate cyclopentanone,cyclohexanone and ethylene glycol monoethyl ether acetate.

The solvents may be present in the overall composition in an amount offrom about 75% to about 98% by weight of the solids in the composition.Solvents, of course, are substantially removed after coating of the topanti-reflective coating on a substrate and drying.

The present invention also provides a process for producingsemiconductor devices using such top anti-reflective coating having avery low level of metal ions, particularly sodium and iron. In thepreferred embodiment, the process utilizes an acidic ion exchange resinto purify the top anti-reflective coating. The subject processcomprises:

a) treating an acidic ion exchange resin with water, preferablydeionized water, followed by a mineral acid solution (e.g. a 5-98%solution of sulfuric, nitric or hydrochloric acid) to reduce the levelof sodium and iron ions in the ion exchange resin to less than 500 ppbeach, preferably less than 200 ppb, more preferably less than 100 ppband most preferably no more than 40 ppb;

b) providing a solution of 5 to 40 weight percent of a water solubleorganic carboxylic acid polymer having a molecular weight of from about500 to about 100,000, preferably from about 1,000 to about 10,000, in asuitable solvent;

c) passing the water soluble organic carboxylic acid polymer solutionthrough the ion exchange resin and reducing the level of sodium and ironions in the solution to less than 200 ppb each, preferably less than 100ppb, more preferably less than 50 ppb, even more preferably less than 20ppb and most preferably less than 10 ppb;

(d) formulating a top anti-reflective coating composition by providingan admixture of:

(1) the treated water soluble organic carboxylic acid;

(2) a fluorine containing water sparingly soluble (0.1%-10% by weight inwater, preferably 0.5%-5% by weight) organic (C₃ -C₁₃ ) aliphaticcarboxylic acid;

(3) an ammonium hydroxide; and

(4) a suitable solvent.

Prior to formulating the top anti-reflective coating composition, asolution of the fluorine containing sparingly water soluble organicaliphatic carboxylic acid in a suitable solvent is preferably passedthrough the ion exchange resin and the level of sodium and iron ions inthe solution reduced to less than 200 ppb each, preterably less than 100ppb, more preferably less than 50 ppb, even more preferably less than 20ppb and most preferably less than 10 ppb.

The solvents for the water soluble organic carboxylic acid, thesparingly water soluble halogen containing organic aliphatic carboxylicacid and the top anti-reflective coating are preferably deionized, e.g.deionized water or deionized diglyme or mixture of deionized water anddeionized diglyme.

Prior to formulating the final top anti-reflective coating, preferablyan admixture is provided of:

(1) the treated water soluble organic carboxylic acid;

(2) the treated halogen containing sparingly water-soluble organicaliphatic carboxylic acid; and

(3) a suitable solvent.

The admixture is then passed through the ion exchange resin and thelevel of sodium and iron ions in the solution reduced to less than 100ppb each, preferably less than 50 ppb, more preferably less than 20 ppband most preferably less than 10 ppb. The ammonium hydroxide is thenadded to the admixture to provide a top antireflective coating having avery low level of metal ions.

The prepared top anti-reflective coating is then applied to a suitablesubstrate by any conventional method used in the photoresist art,including dipping, spraying, whirling and spin coating. When spincoating, for example, the photoresist solution can be adjusted withrespect to the percentage of solids content, in order to provide coatingof the desired thickness, given the type of spinning equipment utilizedand the amount of time allowed for the spinning process. Suitablesubstrates include silicon, aluminum, polymeric resins, silicon dioxide,doped silicon dioxide, silicon nitride, tantalum, copper, polysilicon,ceramics, aluminum/copper mixtures; gallium arsenide and other suchGroup III/V compounds.

The top anti-reflective coatings produced by the described procedure areparticularly suitable for application to thermally grown silicon/silicondioxide-coated wafers, such as are utilized in the production ofmicroprocessors and other miniaturized integrated circuit components. Analuminum/aluminum oxide wafer can also be used. The substrate may alsocomprise various polymeric resins, especially transparent polymers suchas polyesters. The substrate may have an adhesion promoted layer of asuitable composition, such as one containing hexa-alkyl disilazane.

The top anti-reflective coating is coated onto the substrate over thephotoresist composition, and the substrate is treated at a temperaturefrom about 70° C. to about 110° C. for from about 30 seconds to about180 seconds on a hot plate or for from about 15 to about 90 minutes in aconvection oven. This temperature treatment is selected in order toreduce the concentration of residual solvents in the photoresist andanti-reflective coating, while not causing substantial thermaldegradation of the photosensitizer. In general, one desires to minimizethe concentration of solvents and this first temperature treatment isconducted until substantially all of the solvents have evaporated and athin coating of photoresist composition, on the order of one micron inthickness, remains on the substrate. In a preferred embodiment thetemperature is from about 85° C. to about 95° C. The treatment isconducted until the rate of change of solvent removal becomes relativelyinsignificant. The temperature and time selection depends on thephotoresist properties desired by the user, as well as the equipmentused and commercially desired coating times. The coated substrate canthen be exposed to actinic radiation, e.g., ultraviolet radiation, at awavelength of from about 300 nm to about 450 nm, x-ray, electron beam,ion beam or laser radiation, in any desired pattern, produced by use ofsuitable masks, negatives, stencils, templates, etc.

The substrate is then optionally subjected to a post exposure secondbaking or heat treatment either before or after development. The heatingtemperatures may range from about 90° C. to about 120° C., morepreferably from about 100° C. to about 110° C. The heating may beconducted for from about 30 seconds to about 2 minutes, more preferablyfrom about 60 seconds to about 90 seconds on a hot plate or about 30 toabout 45 minutes by convection oven.

The exposed photoresist-coated substrates are then developed to removethe image-wise exposed areas such as by immersion in an alkalinedeveloping solution or developed by spray development process. Thesolution is preferably agitated, for example, by nitrogen burstagitation. The substrates are allowed to remain in the developer untilall, or substantially all, of the photoresist coating has dissolved fromthe exposed areas. Developers may include aqueous solutions of ammoniumhydroxides. One preferred hydroxide is tetramethyl ammonium hydroxide.After removal of the coated wafers from the developing solution, one mayconduct an optional post-development heat treatment or bake to increasethe coating's adhesion and chemical resistance to etching solutions andother substances. The post-development heat treatment can comprise theoven baking of the coating and substrate below the coating's softeningpoint. In industrial applications, particularly in the manufacture ofmicrocircuitry units on silicon/silicon dioxide-type substrates, thedeveloped substrates may be treated with a buffered, hydrofluoric acidbase etching solution.

The following specific examples will provide detailed illustrations ofthe methods of producing and utilizing compositions of the presentinvention. These examples are not intended, however, to limit orrestrict the scope of the invention in any way and should not beconstrued as providing conditions, parameters or values which must beutilized exclusively in order to practice the present invention.

Example 1

1000 grams of a 7 weight percent solution of polyacrylic acid indeionized water was passed through a column of Amberlyst 15 ion exchangeresin which had been cleaned with deionized water, 10% sulfuric acid andthen sufficient deionized water to remove the sulfuric acid. Theuntreated polyacrylic acid solution had a metal ion content as follows:6800 ppb sodium, 1200 ppb potassium, 400 ppb calcium, <10 ppb iron and<10 ppb aluminum. The solution treated according to the procedure ofExample 2 was sampled after, respectively, 179 grams, 330 grams and 525grams of the polyacrylic acid solution had been passed through theAMBERLYST resin column. The treated samples had a very low level ofmetal ions as follows:

    ______________________________________                                        SAMPLE  179 GRAMS    330 GRAMS  525 GRAMS                                     ______________________________________                                        Sodium   20 ppb      <10 ppb    <10 ppb                                       Potassium                                                                             <20 ppb      <20 ppb    <20 ppb                                       Calcium <20 ppb      <20 ppb    <20 ppb                                       Iron    <10 ppb      <10 ppb    <10 ppb                                       Aluminum                                                                              <10 ppb      <10 ppb    <10 ppb                                       ______________________________________                                    

Example 2

17 pounds of AMBERLYST 15 ion exchange resin beads which were rinsedwith deionized water were placed in a 0.45 cubic foot canister. Thecanister was connected through a pump to a drum with a stainless steeltube. 25 gal. of 10 percent sulfuric acid was passed through thecanister using a pump, at a rate of 0.35 gal. per minute. 200 gal. ofdeionized water was passed through the canister at the same rate toremove the sulfuric acid until the pH of the effluent is equal to the pHof the deionized water. 200 gal. of a 10 weight percent polyacrylic acidsolution in deionized water was prepared. The solution had a sodium ionlevel of 360 ppb, an iron level of 190 ppb, a potassium ion level of 600ppb, a chromium ion level of 20 ppb and a calcium ion level of 2600 ppband was passed through the resin canister at the same rate and collectedin a clean drum. The polyacrylic acid solution obtained had a very lowlevel of metal ions as follows: sodium - 93 ppb, iron - 20 ppb,potassium - 13 ppb, calcium - 74 ppb and chromium - 9 ppb.

Example 3

The treated polyacrylic acid solution of Example 2 was again passedthrough the resin canister of Example 2, according to the procedure ofExample 2. The polyacrylic acid solution obtained had an even lowerlevel of metal ions as follows: sodium - 11 ppb, iron - 5 ppb,potassium - 5 ppb, calcium - 34 ppb and chromium - 5 ppb.

Example 4

A solution of 4.0 weight percent of pentadecafluorooctanoic acid indeionized water was prepared. The solution was passed through the resincanister of Example 1, according to the procedures of Example 1. Thepentadecafluorooctanoic acid solution obtained had a low level of metalions as follows: sodium--less than 10 ppb and iron--less than 10 ppb.

Example 5

A solution was prepared from 3.35 weight percent of treatedpentadecafluorooctanoic acid of Example 4, 1.65 weight percent of thetreated polyacrylic acid of Example 2, 1.0 weight percent oftetramethylammonium hydroxide and 94.0 weight percent deionized water.The anti-reflective coating obtained had a low level of metal ions asfollows: sodium--<10 ppb and iron--<20 ppb.

The coating was capable of forming a 717A° film at 4000 RPM with arefractive index of 1.41 for the coated film.

We claim:
 1. A method for producing a top anti-reflective coating composition having a very low level of metal ions consisting essentially of:a) treating an acidic ion exchange resin with deionized water, washing said ion exchange resin with a mineral acid solution and thereby reducing the level of sodium and iron ions in the ion exchange resin to less than 200ppb each; b) providing a solution of 5 to 40 weight percent of a water soluble organic carboxylic acid polymer having a molecular weight of from about 500 to about 100,000 in a suitable solvent; c) rinsing the ion exchange resin with a solvent which is the same as or at least compatible with the solvent for the .component or mixture of components which is to be treated with the ion exchange resin and thereby substantially removing water and saturating the ion exchange resin with said solvent; d) passing the water soluble organic carboxylic acid polymer solution through the ion exchange resin and thereby reducing the level of sodium and iron ions in the solution to less than 100 ppb each; e) formulating a top anti-reflective coating composition by providing an admixture of:(1) the treated water soluble organic carboxylic acid polymer; (2) a fluorine containing water sparingly soluble organic C₃ -C₃₁ aliphatic carboxylic acid; (3) an ammonium hydroxide; and (4) a suitable solvent.
 2. The method of claim 1 wherein, prior to formulating the top anti-reflective coating composition, a solution of the fluorine containing sparingly water soluble organic aliphatic carboxylic acid in a suitable solvent is passed through an acidic ion exchange resin and the level of sodium and iron ions in the solution is reduced to less than 200 ppb each.
 3. The method of claim 2 wherein, prior to formulating the final top anti-reflective coating, an admixture is provided of:(1) the treated water soluble organic carboxylic acid polymer; (2) the treated fluorine containing sparingly water-soluble organic aliphatic carboxylic acid; and (3) a suitable solvent; and(a) the admixture is then passed through the acidic ion exchange resin and the level of sodium and iron ions in the solution is thereby reduced to less than 100 ppb each; (b) the ammonium hydroxide is then added to the admixture to provide a top anti-reflective coating having a very low level of metal ions.
 4. The method of claim 1 wherein the level of sodium and iron ions in the ion exchange resin is reduced to less than 100 ppb each.
 5. The method of claim 4 wherein the level of sodium and iron ions in the ion exchange resin is reduced to no more than 40 ppb each.
 6. The method of claim 1 wherein the sodium and iron ion level in the top anti-reflective-coating composition is reduced to less than 50 ppb each.
 7. The method of claim 1 wherein the sodium and iron ion level in the top-anti-reflective coating composition is reduced to less than 20 ppb each.
 8. The method of claim 1 wherein the ion exchange resin has a sodium and iron ion level of less than 100 ppb each and the resulting anti-reflective coating composition solution has a sodium and iron level of less than 50 ppb each.
 9. The method of claim 1 wherein the ion exchange resin has a sodium and iron ion level of less than 40 ppb each and the resulting top anti-reflective coating has a sodium and iron ion level of less than 20 ppb each. 